(i) Field of the Invention
The present invention relates to a method for preparing amines, and more specifically, it relates to a method for preparing methylamines from methanol and ammonia in which monomethylamine and dimethylamine are obtained in larger amounts, while the production of trimethylamine is inhibited to a sufficiently low level. The methylamines obtained by the method of the present invention are useful as raw materials for the manufacture of solvents and various intermediates of organic synthesis.
(ii) Description of the Prior Art
Methylamines, i.e., monomethylamine, dimethylamine and trimethylamine have been prepared by a method in which methanol or a mixture of methanol and dimethyl ether is reacted with ammonia, a method in which prussic acid is subjected to catalytic reduction, or the like.
These methylamines are produced as a mixture of monomethylamine, dimethylamine and trimethylamine, and they have independent applications, respectively. On the other hand, the demand of these methylamines is localized on dimethylamine and monomethylamine, and so trimethylamine is nowadays in little demand. The methylamines obtained by the reaction of methanol and ammonia in the presence of a conventional amorphous silica-alumina catalyst contain trimethylamine as a main component, and the process using this catalyst has the drawback that the production of dimethylamine which is in great demand is insufficient.
In order to overcome this drawback, U.S. Pat. No. 3,384,667 has suggested a technique in which a dehydrated crystalline aluminosilicate (a zeolite) having a pore diameter of 5-10 .ANG. is used as a catalyst in the reaction of an alcohol having 1 to 18 carbon atoms with ammonia to predominantly produce monoamine and diamine over triamine. Furthermore, as zeolites suitable for the above-mentioned reaction, natural zeolites and synthetic zeolites are recited. That is, this U.S. patent discloses that examples of the desirable natural zeolites include faujasite, analcite, clinoptilolite, ferrierite, chabazite, gmelinite, levynite, erionite and mordenite. It is also disclosed that examples of the desirable synthetic zeolites include X type, Y type and A type zeolites.
There are known a method which comprises mixing methanol with ammonia in a specific ratio, and then carrying out the reaction in the presence of a catalyst such as mordenite to form monomethylamine in a surprisingly large amount (Japanese Patent Application Laid-open No. 113747/1981), and a method which comprises disproportionating monomethylamine on a crystalline aluminosilicate selected from Na mordenites to prepare dimethylamine with a high selectivity (Japanese Patent Application Laid-open No. 46846/1981).
In addition, there are also known a method in which a natural mineral is used as a mordenite in the same manner as in the above-mentioned U.S. Pat. No. 3,384,667 (Japanese Patent Application Laid-open No. 169444/1982), a method in which a mordenite ion-exchanged with lanthanum ions is used as the catalyst (Japanese Patent Application Laid-open No. 49340/1983), a method in which a mordenite containing an ion-exchanged alkaline metal in a specifically limited amount range is used as the catalyst (Japanese Patent Application Laid-open No. 210050/1984), a method in which a steam-treated mordenite is used as the catalyst (Japanese Patent Application Laid-open No. 227841/1984), a method in which an A type zeolite having a low binder content is used as the catalyst (Japanese Patent Application Laid-open No. 69846/1983), and a method in which a Rho type (ZK-5) zeolite is used as the catalyst.
When the zeolite catalyst is used by any of the above-mentioned methods, the production of trimethylamine can be inhibited, but for the purpose of inhibiting the production of trimethylamine to zero or substantially zero, some methods are also known in which a mordenite having pores treated by CVD (chemical vapor deposition) of silicon tetrachloride is used as the catalyst (Japanese Patent Application Laid-open No. 262540/1991; J. Catal., Vol. 131, pp. 482 (1991); and U.S. Pat. No. 5,137,854). Another method is also present in which chabazite, erionite, ZK-5 or a Rho type zeolite deposited or modified with compounds of silicon, aluminum, phosphorus or boron is used as the catalyst to restrict the production of trimethylamine (Japanese Patent Application Laid-open No. 254256/1986, and U.S. Pat. No. 4,683,334). Moreover, there is also known a method which comprises reacting an alcohol with ammonia by the use of SAPO of a non-zeolite molecular sieve as the catalyst to obtain alkylamines (Japanese Patent Application Laid-open No. 734/1990).
As described above, the production of trimethylamine which is in little demand can be inhibited to a low level by using any of the various already disclosed zeolite-based catalysts in the reaction of methanol and ammonia, so that the production of dimethylamine which is in great demand can be increased. However, even if the zeolite-based compound is used as the catalyst, the production ratio of trimethylamine can be merely restricted to usually 10%, or at most about 5%. If the production of trimethylamine can be inhibited to about 1 to 3%, it is not necessary to recycle trimethylamine which is in little demand to the reaction system for disproportionation, and as a result, a manufacturing process can be simplified and utilities, steam or the like, to be used can be decreased.
As a method for decreasing the production of trimethylamine to several percent, there is the above-mentioned method disclosed in Japanese Patent Application Laid-open No. 262540/1991 (U.S. Pat. No. 5,137,854) in which a mordenite deposited with silicon tetrachloride by a CVD treatment is used as the catalyst. However, this method can be easily carried out on a laboratory scale, but it is difficult to industrially manufacture a large amount of the catalyst for use in preparing methylamines.
In the above-mentioned method in which ZK-5 or a Rho type zeolite treated with silicon, aluminum, phosphorus or a boron compound is used to inhibit the production of trimethylamine to several percent (U.S. Pat. No. 4,683,334), there is the drawback that a specific synthetic zeolite such as ZK-5 or Rho is required to be used.